1. Field of the Invention
The present invention relates generally to electrical wiring devices, and particularly to electrical wiring devices having protective features.
2. Technical Background
Examples of electric circuit protection devices include ground fault circuit interrupters (GFCIs), arc fault circuit interrupters (AFCIs), or devices that include both GFCIs and AFCIs in one protective device. An electric circuit typically includes at least one protection device disposed in the breaker box, in a duplex receptacle, in an electrical plug, or the like. The most common fault conditions are ground faults and arc faults. The function of a protection device is to detect the fault and then remove power to the load circuit to substantially eliminate the possibility of shock or fire.
An arc fault is a discharge of electricity between two or more conductors. There are two types of arc faults. One type is a parallel arc fault, and the other is known as a series arc fault. A parallel arc fault is caused by damaged insulation on the hot line conductor or neutral line conductor, or on both the hot line conductor and the neutral line conductor, such as from an overdriven staple. The damaged insulation may cause an arc between the two conductors and may result in a fire. A series arc may be caused by a break in the line or neutral conductors of the electrical distribution system or by a loose wiring device terminal. An arc fault usually manifests itself as a high frequency current signal that typically exhibits a concentration of energy in certain frequency bands. As such, AFCIs may be configured to detect arc faults by being designed to recognize the aforementioned high frequency signature.
A ground fault, on the other hand, is a condition that occurs when a current carrying (hot) conductor contacts ground to create an unintended current path. The unintended current path represents an electrical shock hazard. A ground fault may also represent a fire hazard. A ground fault may occur for several reasons. If the wiring insulation within a load circuit becomes damaged, the hot conductor may contact ground, creating a shock hazard for a user. A ground fault may also occur when equipment comes in contact with water. A ground fault may also be caused by damaged insulation within the facility.
Under normal operating conditions, the current flowing in the hot conductor should equal the current in the neutral conductor. A ground fault upsets this balance and creates a differential current between the hot conductor and the neutral conductor. GFCIs exploit this phenomenon by comparing the current in the hot conductor(s) to the return current in the neutral conductor. In other words, a ground fault is typically detected by sensing the differential current between the two conductors. Upon detecting a ground fault, the GFCI may respond by actuating an alarm and/or interrupting the circuit.
A grounded neutral condition is another type of fault condition that occurs when the load neutral terminal, or a conductor connected to the load neutral terminal, becomes grounded. While this condition does not represent an immediate shock hazard, it is nonetheless an insidious double-fault condition that may lead to a serious injury or a fatality. The reasons for this become apparent when one considers that GFCIs are configured to trip when the differential current is greater than or equal to approximately 6 mA. However, when the load neutral conductor is grounded the GFCI becomes de-sensitized because some of the return path current is diverted to ground. Under these conditions, it may take up to 30 mA of differential current before the GFCI trips. Accordingly, when a fault occurs in a grounded neutral state, the GFCI may fail to trip, exposing a user to experience serious injury or death. There are other reasons why a protective device may fail to perform its function.
The protective device includes electronic and mechanical components that may experience an end-of-life (EOL) condition. For example, protective devices must include some type of fault sensor and detector. The detector output is coupled to an electronic switch. When the switch is turned ON a solenoid is energized. The energized solenoid drives a circuit interrupter in turn. Of course, the circuit interrupter disconnects the load terminals from the line terminals when a fault is detected. Component failure may occur for a variety of reasons. Failure may occur because of the normal aging of electronic components. Mechanical parts may become corroded, experience mechanical wear, or fail because of mechanical abuse. Devices may also fail when they are overloaded when installed. Electrical power surges, such as from lightning, also may result in failure. If any of the sensor, the detector, the switch, solenoid, and/or power supply fail, i.e., an EOL condition is extant, the GFCI may fail to trip, exposing a user to experience serious injury or death. There are other reasons why a protective device may fail to perform its function. Accordingly, a protection device that denies power to a load circuit in the event of an EOL condition is desirable.
In one approach that has been considered, a protective device is equipped with a manually activated test button for determining the operating condition of the device. If the test fails the circuit interrupter permanently disconnects the load terminals from the line terminals. One drawback to this approach relates to the fact that the device only reacts to a problem if the user activates the test button. As such, this approach does not address the aforementioned EOL scenario. Another drawback to this relates to the fact that even if the device is manually tested, an inoperative circuit interrupter allows a fire or shock hazard to persist indefinitely.
In another approach that has been considered, a protective device may be equipped with an automatic test feature. In this approach, the automatic test mechanism periodically tests the device without user intervention. A failed test automatically causes the circuit interrupter to permanently disconnect the load terminals from the line terminals. The drawback to this approach is similar to the manual approach described above. The auto-test device also provides unprotected power to the load circuit when the circuit interrupter is experiencing an EOL condition.
Accordingly, a protective device is needed having a test feature for detecting failure of both electrical components and electro-mechanical components. Further, what is needed is a device having a separate test mechanism configured to deny power to a load circuit in response to the aforementioned EOL conditions.